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1 Kernel Crypto API Architecture
2 ==============================
3
4 Cipher algorithm types
5 ----------------------
6
7 The kernel crypto API provides different API calls for the following
8 cipher types:
9
10 - Symmetric ciphers
11
12 - AEAD ciphers
13
14 - Message digest, including keyed message digest
15
16 - Random number generation
17
18 - User space interface
19
20 Ciphers And Templates
21 ---------------------
22
23 The kernel crypto API provides implementations of single block ciphers
24 and message digests. In addition, the kernel crypto API provides
25 numerous "templates" that can be used in conjunction with the single
26 block ciphers and message digests. Templates include all types of block
27 chaining mode, the HMAC mechanism, etc.
28
29 Single block ciphers and message digests can either be directly used by
30 a caller or invoked together with a template to form multi-block ciphers
31 or keyed message digests.
32
33 A single block cipher may even be called with multiple templates.
34 However, templates cannot be used without a single cipher.
35
36 See /proc/crypto and search for "name". For example:
37
38 - aes
39
40 - ecb(aes)
41
42 - cmac(aes)
43
44 - ccm(aes)
45
46 - rfc4106(gcm(aes))
47
48 - sha1
49
50 - hmac(sha1)
51
52 - authenc(hmac(sha1),cbc(aes))
53
54 In these examples, "aes" and "sha1" are the ciphers and all others are
55 the templates.
56
57 Synchronous And Asynchronous Operation
58 --------------------------------------
59
60 The kernel crypto API provides synchronous and asynchronous API
61 operations.
62
63 When using the synchronous API operation, the caller invokes a cipher
64 operation which is performed synchronously by the kernel crypto API.
65 That means, the caller waits until the cipher operation completes.
66 Therefore, the kernel crypto API calls work like regular function calls.
67 For synchronous operation, the set of API calls is small and
68 conceptually similar to any other crypto library.
69
70 Asynchronous operation is provided by the kernel crypto API which
71 implies that the invocation of a cipher operation will complete almost
72 instantly. That invocation triggers the cipher operation but it does not
73 signal its completion. Before invoking a cipher operation, the caller
74 must provide a callback function the kernel crypto API can invoke to
75 signal the completion of the cipher operation. Furthermore, the caller
76 must ensure it can handle such asynchronous events by applying
77 appropriate locking around its data. The kernel crypto API does not
78 perform any special serialization operation to protect the caller's data
79 integrity.
80
81 Crypto API Cipher References And Priority
82 -----------------------------------------
83
84 A cipher is referenced by the caller with a string. That string has the
85 following semantics:
86
87 ::
88
89 template(single block cipher)
90
91
92 where "template" and "single block cipher" is the aforementioned
93 template and single block cipher, respectively. If applicable,
94 additional templates may enclose other templates, such as
95
96 ::
97
98 template1(template2(single block cipher)))
99
100
101 The kernel crypto API may provide multiple implementations of a template
102 or a single block cipher. For example, AES on newer Intel hardware has
103 the following implementations: AES-NI, assembler implementation, or
104 straight C. Now, when using the string "aes" with the kernel crypto API,
105 which cipher implementation is used? The answer to that question is the
106 priority number assigned to each cipher implementation by the kernel
107 crypto API. When a caller uses the string to refer to a cipher during
108 initialization of a cipher handle, the kernel crypto API looks up all
109 implementations providing an implementation with that name and selects
110 the implementation with the highest priority.
111
112 Now, a caller may have the need to refer to a specific cipher
113 implementation and thus does not want to rely on the priority-based
114 selection. To accommodate this scenario, the kernel crypto API allows
115 the cipher implementation to register a unique name in addition to
116 common names. When using that unique name, a caller is therefore always
117 sure to refer to the intended cipher implementation.
118
119 The list of available ciphers is given in /proc/crypto. However, that
120 list does not specify all possible permutations of templates and
121 ciphers. Each block listed in /proc/crypto may contain the following
122 information -- if one of the components listed as follows are not
123 applicable to a cipher, it is not displayed:
124
125 - name: the generic name of the cipher that is subject to the
126 priority-based selection -- this name can be used by the cipher
127 allocation API calls (all names listed above are examples for such
128 generic names)
129
130 - driver: the unique name of the cipher -- this name can be used by the
131 cipher allocation API calls
132
133 - module: the kernel module providing the cipher implementation (or
134 "kernel" for statically linked ciphers)
135
136 - priority: the priority value of the cipher implementation
137
138 - refcnt: the reference count of the respective cipher (i.e. the number
139 of current consumers of this cipher)
140
141 - selftest: specification whether the self test for the cipher passed
142
143 - type:
144
145 - skcipher for symmetric key ciphers
146
147 - cipher for single block ciphers that may be used with an
148 additional template
149
150 - shash for synchronous message digest
151
152 - ahash for asynchronous message digest
153
154 - aead for AEAD cipher type
155
156 - compression for compression type transformations
157
158 - rng for random number generator
159
160 - givcipher for cipher with associated IV generator (see the geniv
161 entry below for the specification of the IV generator type used by
162 the cipher implementation)
163
164 - kpp for a Key-agreement Protocol Primitive (KPP) cipher such as
165 an ECDH or DH implementation
166
167 - blocksize: blocksize of cipher in bytes
168
169 - keysize: key size in bytes
170
171 - ivsize: IV size in bytes
172
173 - seedsize: required size of seed data for random number generator
174
175 - digestsize: output size of the message digest
176
177 - geniv: IV generation type:
178
179 - eseqiv for encrypted sequence number based IV generation
180
181 - seqiv for sequence number based IV generation
182
183 - chainiv for chain iv generation
184
185 - <builtin> is a marker that the cipher implements IV generation and
186 handling as it is specific to the given cipher
187
188 Key Sizes
189 ---------
190
191 When allocating a cipher handle, the caller only specifies the cipher
192 type. Symmetric ciphers, however, typically support multiple key sizes
193 (e.g. AES-128 vs. AES-192 vs. AES-256). These key sizes are determined
194 with the length of the provided key. Thus, the kernel crypto API does
195 not provide a separate way to select the particular symmetric cipher key
196 size.
197
198 Cipher Allocation Type And Masks
199 --------------------------------
200
201 The different cipher handle allocation functions allow the specification
202 of a type and mask flag. Both parameters have the following meaning (and
203 are therefore not covered in the subsequent sections).
204
205 The type flag specifies the type of the cipher algorithm. The caller
206 usually provides a 0 when the caller wants the default handling.
207 Otherwise, the caller may provide the following selections which match
208 the aforementioned cipher types:
209
210 - CRYPTO_ALG_TYPE_CIPHER Single block cipher
211
212 - CRYPTO_ALG_TYPE_COMPRESS Compression
213
214 - CRYPTO_ALG_TYPE_AEAD Authenticated Encryption with Associated Data
215 (MAC)
216
217 - CRYPTO_ALG_TYPE_BLKCIPHER Synchronous multi-block cipher
218
219 - CRYPTO_ALG_TYPE_ABLKCIPHER Asynchronous multi-block cipher
220
221 - CRYPTO_ALG_TYPE_GIVCIPHER Asynchronous multi-block cipher packed
222 together with an IV generator (see geniv field in the /proc/crypto
223 listing for the known IV generators)
224
225 - CRYPTO_ALG_TYPE_KPP Key-agreement Protocol Primitive (KPP) such as
226 an ECDH or DH implementation
227
228 - CRYPTO_ALG_TYPE_DIGEST Raw message digest
229
230 - CRYPTO_ALG_TYPE_HASH Alias for CRYPTO_ALG_TYPE_DIGEST
231
232 - CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash
233
234 - CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash
235
236 - CRYPTO_ALG_TYPE_RNG Random Number Generation
237
238 - CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher
239
240 - CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of
241 CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression /
242 decompression instead of performing the operation on one segment
243 only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace
244 CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted.
245
246 The mask flag restricts the type of cipher. The only allowed flag is
247 CRYPTO_ALG_ASYNC to restrict the cipher lookup function to
248 asynchronous ciphers. Usually, a caller provides a 0 for the mask flag.
249
250 When the caller provides a mask and type specification, the caller
251 limits the search the kernel crypto API can perform for a suitable
252 cipher implementation for the given cipher name. That means, even when a
253 caller uses a cipher name that exists during its initialization call,
254 the kernel crypto API may not select it due to the used type and mask
255 field.
256
257 Internal Structure of Kernel Crypto API
258 ---------------------------------------
259
260 The kernel crypto API has an internal structure where a cipher
261 implementation may use many layers and indirections. This section shall
262 help to clarify how the kernel crypto API uses various components to
263 implement the complete cipher.
264
265 The following subsections explain the internal structure based on
266 existing cipher implementations. The first section addresses the most
267 complex scenario where all other scenarios form a logical subset.
268
269 Generic AEAD Cipher Structure
270 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
271
272 The following ASCII art decomposes the kernel crypto API layers when
273 using the AEAD cipher with the automated IV generation. The shown
274 example is used by the IPSEC layer.
275
276 For other use cases of AEAD ciphers, the ASCII art applies as well, but
277 the caller may not use the AEAD cipher with a separate IV generator. In
278 this case, the caller must generate the IV.
279
280 The depicted example decomposes the AEAD cipher of GCM(AES) based on the
281 generic C implementations (gcm.c, aes-generic.c, ctr.c, ghash-generic.c,
282 seqiv.c). The generic implementation serves as an example showing the
283 complete logic of the kernel crypto API.
284
285 It is possible that some streamlined cipher implementations (like
286 AES-NI) provide implementations merging aspects which in the view of the
287 kernel crypto API cannot be decomposed into layers any more. In case of
288 the AES-NI implementation, the CTR mode, the GHASH implementation and
289 the AES cipher are all merged into one cipher implementation registered
290 with the kernel crypto API. In this case, the concept described by the
291 following ASCII art applies too. However, the decomposition of GCM into
292 the individual sub-components by the kernel crypto API is not done any
293 more.
294
295 Each block in the following ASCII art is an independent cipher instance
296 obtained from the kernel crypto API. Each block is accessed by the
297 caller or by other blocks using the API functions defined by the kernel
298 crypto API for the cipher implementation type.
299
300 The blocks below indicate the cipher type as well as the specific logic
301 implemented in the cipher.
302
303 The ASCII art picture also indicates the call structure, i.e. who calls
304 which component. The arrows point to the invoked block where the caller
305 uses the API applicable to the cipher type specified for the block.
306
307 ::
308
309
310 kernel crypto API | IPSEC Layer
311 |
312 +-----------+ |
313 | | (1)
314 | aead | <----------------------------------- esp_output
315 | (seqiv) | ---+
316 +-----------+ |
317 | (2)
318 +-----------+ |
319 | | <--+ (2)
320 | aead | <----------------------------------- esp_input
321 | (gcm) | ------------+
322 +-----------+ |
323 | (3) | (5)
324 v v
325 +-----------+ +-----------+
326 | | | |
327 | skcipher | | ahash |
328 | (ctr) | ---+ | (ghash) |
329 +-----------+ | +-----------+
330 |
331 +-----------+ | (4)
332 | | <--+
333 | cipher |
334 | (aes) |
335 +-----------+
336
337
338
339 The following call sequence is applicable when the IPSEC layer triggers
340 an encryption operation with the esp_output function. During
341 configuration, the administrator set up the use of rfc4106(gcm(aes)) as
342 the cipher for ESP. The following call sequence is now depicted in the
343 ASCII art above:
344
345 1. esp_output() invokes crypto_aead_encrypt() to trigger an
346 encryption operation of the AEAD cipher with IV generator.
347
348 In case of GCM, the SEQIV implementation is registered as GIVCIPHER
349 in crypto_rfc4106_alloc().
350
351 The SEQIV performs its operation to generate an IV where the core
352 function is seqiv_geniv().
353
354 2. Now, SEQIV uses the AEAD API function calls to invoke the associated
355 AEAD cipher. In our case, during the instantiation of SEQIV, the
356 cipher handle for GCM is provided to SEQIV. This means that SEQIV
357 invokes AEAD cipher operations with the GCM cipher handle.
358
359 During instantiation of the GCM handle, the CTR(AES) and GHASH
360 ciphers are instantiated. The cipher handles for CTR(AES) and GHASH
361 are retained for later use.
362
363 The GCM implementation is responsible to invoke the CTR mode AES and
364 the GHASH cipher in the right manner to implement the GCM
365 specification.
366
367 3. The GCM AEAD cipher type implementation now invokes the SKCIPHER API
368 with the instantiated CTR(AES) cipher handle.
369
370 During instantiation of the CTR(AES) cipher, the CIPHER type
371 implementation of AES is instantiated. The cipher handle for AES is
372 retained.
373
374 That means that the SKCIPHER implementation of CTR(AES) only
375 implements the CTR block chaining mode. After performing the block
376 chaining operation, the CIPHER implementation of AES is invoked.
377
378 4. The SKCIPHER of CTR(AES) now invokes the CIPHER API with the AES
379 cipher handle to encrypt one block.
380
381 5. The GCM AEAD implementation also invokes the GHASH cipher
382 implementation via the AHASH API.
383
384 When the IPSEC layer triggers the esp_input() function, the same call
385 sequence is followed with the only difference that the operation starts
386 with step (2).
387
388 Generic Block Cipher Structure
389 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
390
391 Generic block ciphers follow the same concept as depicted with the ASCII
392 art picture above.
393
394 For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The
395 ASCII art picture above applies as well with the difference that only
396 step (4) is used and the SKCIPHER block chaining mode is CBC.
397
398 Generic Keyed Message Digest Structure
399 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
400
401 Keyed message digest implementations again follow the same concept as
402 depicted in the ASCII art picture above.
403
404 For example, HMAC(SHA256) is implemented with hmac.c and
405 sha256_generic.c. The following ASCII art illustrates the
406 implementation:
407
408 ::
409
410
411 kernel crypto API | Caller
412 |
413 +-----------+ (1) |
414 | | <------------------ some_function
415 | ahash |
416 | (hmac) | ---+
417 +-----------+ |
418 | (2)
419 +-----------+ |
420 | | <--+
421 | shash |
422 | (sha256) |
423 +-----------+
424
425
426
427 The following call sequence is applicable when a caller triggers an HMAC
428 operation:
429
430 1. The AHASH API functions are invoked by the caller. The HMAC
431 implementation performs its operation as needed.
432
433 During initialization of the HMAC cipher, the SHASH cipher type of
434 SHA256 is instantiated. The cipher handle for the SHA256 instance is
435 retained.
436
437 At one time, the HMAC implementation requires a SHA256 operation
438 where the SHA256 cipher handle is used.
439
440 2. The HMAC instance now invokes the SHASH API with the SHA256 cipher
441 handle to calculate the message digest.